At right is the Beyoncé/L'Oreal ad as it appeared in most magazines; at left is how it appeared in Essence. Did the ad people or publishers darken her skin for the magazine targeted at black women? Quips a blogger at Young, Black & Fabulous: "Maybe the printing machines were just off." But over at Racialicious, there's another theory about the original ad: "When filming hair, incredibly strong lamps are used to make each strand visible and shiny… Given that L'Oréal is selling a haircolor and highlights product, they undoubtedly employed a ton of lights… This is not a case of L'Oréal manipulating Beyoncé via Photoshop (at least not beyond the normal ultra-retouching done for fashion shots). Quite the opposite. L'Oréal should have used Photoshop-to restore the natural skin tone removed by the lighting." [Radar, Young, Black & Fabulous, Racialicious]
Everything in the ad on the left is proportionately more red and yellow than the other image. And if one image is a scan and the other is a jpg file then likely you're going to see some variation in color.
That's not to say someone from Loreal's ad agency looked the other way on a press check but I think this is pretty typical of color variations in print.